Briefly, there's new story content, but more importantly the huge World vs. World battles have undergone some treatment. There's no more culling, which is the game hiding people in large battles in order to boost performance. You'll now see everyone.

There's a new WVW progression system as well, based around earning WVW XP and climbing WVW ranks. By doing so you can unlock new titles and abilities for use in WVW.

The release of The Razing also coincides with the launch of leaderboards for Guild Wars 2. These won't be immediately visible because ArenaNet wants to compile lots of stable information before you see them.